1. Field of the Invention
The present invention relates to an air intake device of an internal combustion engine.
2. Description of the Prior Art
In order to create a strong swirl motion in the combustion chamber of an engine when the engine is operating under a light load while preventing the volumetric efficiency from decreasing when the engine is operating under a heavy load at a high speed, an engine having a helically-shaped intake port is known, as disclosed in Japanese Unexamined Patent Publication No. 58-23224. This helically-shaped intake port comprises a helical portion formed around the intake valve, a substantially straight extending inlet passage portion tangentially connected to the helical portion, and a bypass passage branched off from the inlet passage portion and connected to the helix terminating portion of the helical portion. A flow path control valve is arranged in the bypass passage and opened when the amount of air fed into the engine cylinders becomes larger than a predetermined amount. In this engine, when the amount of air fed into the engine cylinders is small and thus the engine is operating under a light load, since the flow path control valve is closed, air is introduced into the helical portion from the inlet passage portion. As a result, the entirety of the air swirls in the helical portion and, thus, a strong swirl motion is created in the combustion chamber. Contrary to this, when the amount of air fed into the engine cylinders is large and, thus, since the engine is operating under a heavy load at a high speed, the flow path control valve is opened. As a result, since the cross-sectional area of the air flow path in the intake port is increased and, in addition, a large part of the air is fed into the engine cylinders via the bypass passage having a small flow resistance, it is possible to ameliorate the reduction of the volumetric efficiency. However, in this engine, when the engine is operating under a heavy load at a high speed, since a part of the air flows within the helical portion having a large flow resistance and, in addition, the flow path control valve provides a flow resistance for the air, it is difficult to obtain a satisfactory high volumetric efficiency.